This invention relates to steel alloys, and, more particularly, to a microalloyed steel that can be hot rolled to reduced sections with a good combination of strength, toughness, and other properties such as fatigue strength.
In the most common approach, the frames of large trucks are constructed from side rails made of steel, which are joined to each other with cross members. The engine, drive train, suspension, and cargo container are then supported on the frame. The assembly must be extremely rugged and capable of operating without failure for extended years of use and in extreme environmental conditions.
The side rails are conventionally made of hot-rolled band material such as a Si-semikilled, modified 1027 steel that is uncoiled, sheared, and cold formed into a structural section of uniform thickness, such as a C-section or channel. Depending upon the steel alloy employed, the as-formed section may be used without further heat treatment, or it may be heat treated to realize the full potential of the mechanical properties.
The manufacturers of truck frames have developed or are developing specifications to be met by the steel alloys used in the side rails. The specifications usually involve standards for mechanical properties such as yield strength, ultimate tensile strength, notch toughness transition temperature, and/or fatigue strength. The supplier of the steel must provide a steel that meets the specifications at as low a cost of production as possible, in order to be competitive.
There have generally been two approaches to meeting the specifications currently set for side rails by major truck manufacturers. In one, a steel having a relatively small amount of alloying additions is hot rolled, cold formed, and heat treated. In the other, a steel having more alloying additions is hot rolled and cold formed, with no heat treatment required because the alloying additions produce sufficiently good mechanical properties in this condition that the standards are met without heat treatment. The heat-treated steels typically have higher strength and toughness levels than the as-rolled alloyed steels, but the heat treatment raises the cost of the final product.
A possible future development is the use of side rails that are not of uniform thickness. Specifically, the flanges (sides) would have a greater thickness than the web that connects the flanges. Variable thickness sections would provide an increased section modulus, and thence increased payload (or, alternatively, decreased weight and fuel consumption), and reduced vehicle size. Such a variable-thickness section would be hot rolled, and, with today's steels, heat treated in an attempt to achieve the required mechanical properties. As indicated, the heat treatment would substantially increase the product cost, negating some of the advantages that would otherwise be achieved.
There is a need for a steel that meets the requirements of truck manufacturers at as low a production cost as possible. Such a steel would desirably be suitable for forming hot rolled sections of variable thickness and meeting property requirements without heat treating. The present invention fulfills this need, and further provides related advantages.